The invention relates to a method to make vapor-liquid-solid (VLS) silicon carbide whisker reinforced metal matrix composites by a squeeze casting process, which composites have a high tensile strength and elastic modulus with low density.
There is a good deal of interest and desire to produce strong composite materials reinforced with inorganic fibers. Research is being directed to employing inorganic fibers such as silica, silicon carbide, alumina, carbon or boron as the reinforcing material with a metal such as aluminum, magnesium, copper, nickel or titanium to form a composite.
Accordingly, there is a desire to develop a process to produce fiber or whisker reinforced metal matrix composites whereby the composites so produced have nearly the theoretically predicted increase in strength and elastic modulus and are not weakened through damage to, or deterioration of, the fibers during processing.
Various processes have been tried, including low pressure casting methods, however, these produce porous composites; powder-metallurgical methods employing heat and pressure, however, the brittle fibers are damaged or broken during blending and pressing; methods of infiltrating the fibers such as a yarn or tow with molten metal, however, the composites have numerous voids; high-pressure solidification casting, however, the high initial pressure during infiltration results in fiber breakage and/or preform damage; coating each fiber, however, this process is laborious and not very practical; and plasma spraying of metal particles onto the fibers, however, this method will not provide infiltration of a body of fibers. U.S. Pat. No. 3,695,335 describes a method using an encapsulation pressure process. U.S. Pat. No. 4,526,841 describes another method adding specific alloying elements to the metal matrix to increase the mechanical strength of the composite.
In accordance with this invention, it has been found that the strength of fiber reinforced metal matrix composites are increased by employing VLS silicon carbide whiskers as the reinforcing material with metals. Further, in accordance with this invention, it has been found that a squeeze casting process to produce the VLS silicon carbide whisker reinforced metal matrix composites does not damage the whisker reinforcements and results in a composite with increased strength, good bonding and negligible porosity. The improvements in the metal matrix composites appear to be a result of the two stage pressure cycle used in the squeeze casting process. A low pressure is used to infiltrate the whiskers so that there is minimal whisker breakage. The pressure is then increased following infiltration and held during solidification of the composite, resulting in negligible solidification shrinkage porosity in the composite.
It is an object of the instant invention to provide VLS silicon carbide whisker reinforced metal matrix composites with high tensile strength and elastic modulus with low density. It is another object of the instant invention to provide VLS silicon carbide whisker reinforced metal matrix composites by a squeeze casting process.
These and other objects, together with the advantages over known methods shall become apparent from the specification which follows and are accomplished by the invention as hereinafter described and claimed.